It is possible to create a child ticket against a 'closed' parent ticket.

[Mark Ryan]

Two things to do here:

• If a (potential) parent ticket is closed, then deactivate the 'create child' buttons.
• When a new ticket is created, run a check to see if the parent is indeed still an active ticket. (This accounts for the case where someone might try and raise a ticket 'manually' instead of via a parent ticket).

[Mark Ryan]

(In [10308]) fixes #8731

• Ticket creation buttons are 'disabled' if the ticket itself is 'closed'.
• The plugin now queries the parent ticket to check it is not 'closed' and disallows if it is.
• This is only implemented for the 0.12 version of the plugin.